Unlike inorganic semiconductor light emitting devices, polymer light emitting devices are generally simple and relatively easy and inexpensive to fabricate. Also, a variety of colors and large-area devices are easily attained. However, one major problem in prior art polymers used in light emitting devices is that most of them have relative low quantum efficiency. For example. poly(phenylene vinylene) (PPV), one of the most studied polymers for LED application, has electroluminescence (EL) quantum efficiency approximately 0.06% with magnesium as n-contact metal electrode. Therefore the challenge is to improve the fluorescent efficiencies of polymers so that polymer light emitting devices can be practically useful.
Since the first report of electroluminescence in organic polymer, specifically poly(phenylene vinylene) (PPV), a variety of polymers have been shown to exhibit electroluminescence. However, it has been found necessary in most case to use metals with low work functions, such as calcium, magnesium, lithium, as the electron injecting contact in order to achieve good electroluminescent efficiencies. Since metals with low work functions are normally susceptible to atmospheric degradation, and are difficult to process and encapsulate, it would be of great advantage to be able use metals with higher work functions, such as aluminum, which can be easily processed and have much better ambient stability, as electron injecting contacts.
In 1993, a group from the University of Cambridge reported that light emitting devices fabricated from several soluble derivatives of poly(cyanoterephthalydene) (CN-PPV), dialkoxy CN-PPVs, have internal electroluminescent quantum efficiency of 4%. More importantly, LEDs with metals of higher work functions as electron injecting contacts, such as aluminum, were found to be as efficient as electron injecting contacts as those with metals of low work functions, such as magnesium, calcium, etc. The high electron affinities of the dialkoxy CN-PPVs, which make the injection of electrons from aluminum possible, are being credited for the marked improvement.
Though the above prior art represents an important step in the development of polymer light emitting devices with stable contacts, the emission of the prior art polymer with a peak around 710 nm is in a region of the visible spectrum less sensible to human eyes. Therefore the prior art polymer is hardly useful for display purpose.
The problem could be partially solved by using the pristine CN-PPV, which has even higher electron affinity, yet lower emission wavelength that is more desirable for display applications.
The preparation of CN-PPV was first disclosed in 1960 by Lenz and coworker in their search for polymers that have the thermal stability of polytetrafluoroethylene (Teflon). Though CN-PPV is thermally stable up to 500.degree. C., the lack of convenient processability, due to its insolubility and infusibility, has hindered its development for any practical applications.
It is a purpose of this invention to provide a new precursor route method for the preparation of poly(phenylene vinylene) and its derivatives.
It is another purpose of this invention to provide a class of new processable precursors to CN-PPV for use in light emitting devices.
It is still another purpose of this invention to provide a preparation method for a class of new processable precursors to CN-PPV for use in light emitting devices.
It is a further purpose of this invention to provide a class of new processable precursors to CN-PPV for use in light emitting devices with stable electron injecting contacts.